Why are you worried about winterizing your tank flush system? Doesn't this valve drain once the water source is removed? This has been discussed on the Airstream forum and the agreement is that the flush system does not need to be winterized. I have never done mine and have not had a problem.
It is suggested that the pressure should not be more than 60 psi. There are two arguments here. If it is more than 60 you can damage fittings in the trailer. On the other hand, if you have a built in pressure reducer you should be good to go. The pressure reducer works regardless of it is water pressure or air pressure.
I have never heard about any problems using a compressor that uses oil. This has never been discussed regarding "oil taste" in the water lines. Even if this was true, why would it matter when blowing out your black water rinse line? This confuses me.
